Radial piston pump for providing high pressure in fuel injection systems of internal combustion engines

ABSTRACT

The invention relates to a radial piston pump ( 1 ) for high-pressure fuel generation in fuel injection systems of internal combustion engines, in particular in a common rail injection system, having a drive shaft ( 4 ) which is mounted in a pump casing ( 2 ) and has an eccentric shaft section ( 6 ) on which a running roller ( 8 ) is mounted, and having preferably a plurality of pistons ( 16 ), which are arranged in a respective cylinder ( 14 ) radially with respect to the drive shaft ( 4 ) and each have a piston footplate ( 18 ), which makes contact with the circumferential surface ( 10, 12 ) of the running roller ( 8 ), at their ends facing the running roller ( 8 ).  
     The invention provides that at least that surface ( 28 ) of the piston footplate ( 18 ) which is in contact with the circumferential surface ( 10, 12 ) of the running roller ( 8 ) consists of a wear-resistant material, namely of hard metal, a ceramic material, a cast carbide material, or cermet.

The invention is based on a radial piston pump for high-pressure fuelgeneration in fuel injection systems of internal combustion engines, inparticular in a common rail injection system, having a drive shaft whichis mounted in a pump casing and has an eccentric shaft section on whicha running roller is mounted, and having preferably a plurality ofpistons, which are arranged in a respective cylinder radially withrespect to the drive shaft and each have a piston footplate, which makescontact with the circumferential surface of the running roller, at theirends facing the running roller, in accordance with the preamble of claim1.

A radial piston pump of this type is known, for example, from DE 198 09315 A1. The piston footplate and the running roller of the known radialpiston pump generally consist of case-hardened steel or of heat-treatedsteel. Over the course of time, however, sliding wear to thesecomponents can occur as a result of adhesion, abrasion or surfacespalling. This undesirable wear can lead to failure of the radial pistonpump and therefore also to failure of the internal combustion engine.

By contrast, the present invention is based on the object of furtherdeveloping a radial piston pump of the type described in theintroduction in such a manner as to increase its reliability.

This object is achieved according to the invention by the characterizingfeatures of claim 1.

The susceptibility of the piston footplate/running roller slidingpairing to wear is significantly reduced by virtue of the fact that, forthe first time, at least that surface of the piston footplate which isin contact with the circumferential surface of the running rollerconsists of a wear-resistant material, namely of hard metal, a ceramicmaterial, a cast carbide material or cermet. The materials listed have asignificantly higher modulus of elasticity compared to the steelmaterials used hitherto, which results in reduced deformation under loadand consequently also in a more uniform surface pressure withoutsignificant stress peaks. If ceramic materials are used, in particulartheir lower weight plays an advantageous role, since the pistonfootplate together with the piston is accelerated and decelerated at ahigh frequency, and consequently the mass inertia is significantlyreduced.

The piston footplate may be made entirely from the wear-resistantmaterial, or else it consists, as hitherto, of case-hardened steel orheat-treated steel but bears at least one insert made from thewear-resistant material on its surface facing the running roller. Theuse of inserts brings the advantage of a modular structure, i.e. astandardized piston footplate can be provided with inserts made fromdifferent material, so that numerous variants can be produced.

If a ceramic material is used, this material preferably contains siliconnitride Si₃N₄ and has a surface roughness R_(z) of between 0.15 μm and0.5 μm. Hard metals may consist, for example, of G20, GC37 or GC20 andmay have a surface roughness R_(z) of between 0.3 μm and 1.0 μm, whilethe cast carbide material is formed by a chilled cast iron material, inparticular by GGH or SoGGH, which has a surface roughness R_(z) ofbetween 0.5 μm and 2.0 μm.

It is particularly preferable for the piston footplate, on its surfacefacing the running roller, to have at least two grooves which cross oneanother. This eliminates the overlap region of piston foot disk andrunning roller without a supply of lubricant. Fuel can accumulate in thegrooves, which act as build-up gaps, and this fuel, on account of thesliding velocity between the running roller and the piston footplate,promotes the formation of a hydrodynamic sliding film, which furtherreduces the wear to the sliding surfaces.

Exemplary embodiments of the invention are illustrated in the drawingsand explained in more detail in the description which follows. In thedrawings:

FIG. 1 shows a cross-sectional illustration of a radial piston pump witha piston footplate and a drive shaft in accordance with a firstembodiment of the invention;

FIG. 2 shows a large cross-sectional illustration of a piston and pistonfootplate in accordance with a further embodiment.

FIG. 2 a shows an enlarged excerpt from FIG. 2;

FIG. 2 b shows a further enlarged excerpt from FIG. 2;

FIG. 3 shows a view of the piston footplate from FIG. 2 from below;

FIG. 4 shows a cross-sectional illustration of a piston with pistonfootplate and a drive shaft in accordance with a further embodiment;

FIG. 5 shows a cross-sectional illustration of a drive shaft inaccordance with a further embodiment;

FIG. 6 shows a view on line VI-VI from FIG. 5;

FIG. 7 shows a view on line VII-VII from FIG. 6.

The radial piston pump 1 shown in FIG. 1 is preferably used to generatethe system pressure for the high-pressure reservoir (rail) of a commonrail injection system of a compression-ignition internal combustionengine. It comprises a drive shaft 4 mounted in a pump casing 2 with aneccentric shaft section 6, on which a polygonal running roller 8, whichcan rotate with respect to the shaft section 6, is mounted. Thepolygonal running roller 8 has planar flat sections 12 arranged at acircumferential distance from one another along its circumferentialsurface 10.

The piston footplate 18 of a piston 16 guided radially with respect tothe drive shaft 4 in a cylinder 14 is supported on each of the flatsections 12 of the running roller 8. The piston footplate 18 ispreferably pivotably connected, by means of a spherical bearing 20, tothe end of the piston 16 which faces towards the drive shaft 4. Thespherical bearing 20 is realized, for example, by the end of the pistonbeing designed as a partial ball 22 which engages in a spherical recess24 of complementary design in the piston footplate 18. Furthermore, thepiston footplate 18, together with the piston 16, is prestressed by aspring 26 onto the associated flat section 12 of the running roller 8.The way in which a radial piston pump 1 of this type functions isdescribed, for example, in DE 198 02 475 A1 and therefore will not bedealt with in any further detail here.

At least that surface 28 of the piston footplate 18 which is in contactwith the circumferential surface 10 of the running roller 8 consists ofa wear-resistant material, namely of hard metal, a ceramic material, acast carbide material or cermet. This is preferably realized by virtueof the fact that the piston footplate 18, on its surface 28 facingtowards the running roller 8, has at least one, for example disk-like,insert 30 made from the wear-resistant material. The insert 30 may bepositively and/or cohesively connected to the remaining piston footplate18, for example by adhesive bonding or soldering. The insert 30 may, asshown in FIG. 1, extend over the entire contact surface 28 of the pistonfootplate 18 with the running roller 8 or only over part of this contactsurface. Alternatively, it is also possible for the entire pistonfootplate 18 to be made from the wear-resistant material, so that thereis no need for an additional insert 30.

If a ceramic material is used for the piston footplate 18, it preferablycontains silicon nitride Si₃N₄. Hard metals may, for example, consistsof G20, GC37 or GC20, while the cast carbide material may contain achilled cast iron material, in particular GGH or SoGGH.

Furthermore, the piston 16 itself may be made from wear-resistantmaterial, for example from an Si₃N₄ ceramic or a ZrO₂ ceramic. Thepiston 16 may be produced by extrusion and have a porosity of less than5%, in which case the surface is infiltrated with MOS₂. Alternatively,the piston 16 may also be isostatically pressed and sintered.

Not least, it is also the case that at least part of the running roller8, in particular the flat sections 12, consists of a wear-resistantmaterial, namely of hard metal, a precision-cast material, a castcarbide material, a sintered tool steel or an alloyed nitriding steel.

As in the case of the piston footplate 18, this is preferably realizedby virtue of the fact that the flat sections 12 are each provided withan insert 32 of the wear-resistant material, as shown in FIG. 1. Aninsert 32 of this type is in each case held positively and/or cohesivelyin a recess 34 of complementary shape in the flat section 12, forexample by adhesive bonding or soldering. Alternatively, the entirerunning roller 8 may consist of the wear-resistant material.

If hard metal is used for the inserts 32 or for the running roller 8itself, examples of suitable materials include G20, GC37 and GC20. Asuitable precision-cast material is formed, for example, byGX-210WCr13H, while a suitable cast carbide material is locallyremelted, carbide SoGGH (gradient material). A suitable sintered toolsteel is ASP23. A nitriding steel which has been specially alloyed withCr and/or Mo and/or V and/or C by nitriding or gas-nitriding is used fora variant with a gradient material. The basic elements and the processparameters used in the nitriding lead to deep diffusion with hardnessesof HV 750 to 850 combined, at the same time, with a higher strength ofthe base material. The compound layer which is formed is removed bygrinding for functional reasons.

The surfaces of the piston footplate 18 and of the running roller 8preferably have a surface roughness R_(z) of between 0.15 μm and 2 μm,depending on the materials used, on the sliding surfaces. The lowerlimit applies to ceramic, in particular a range from 0.15 μm to 0.5 μm,while the upper limit applies to metals such as SoGGH or ASP23. Asurface roughness R_(z) of between 0.3 μm and 1 μm is provided for hardmetal.

The table below lists preferred material pairings for the pistonfootplate 18, on the one hand, and the running roller 8, on the otherhand. If inserts are used both in the running roller 8 and in the pistonfootplate 18, any desired combinations of material pairings are possiblewith the support bodies in each case unchanged. In particular, with thepairings in the table in which the running roller 8 preferably consistsentirely of the wear-resistant material (“solid material”), it isalternatively also possible to use inserts 32 made from thecorresponding material in the region of the flat sections 12, as hasalready been demonstrated in FIG. 1. The running roller 8 as supportbody for the inserts 32 may then consist of a different material, forexample 50Cr4, 42CrV4 or 16MnCr5.

The exemplary embodiment in the third line of the table plays aparticular role. In this case, a carbide zone is in each case formed inthe region of the flat sections 12 of the running roller 8 consisting ofa cast steel material and illustrated separately in FIG. 5. This carbidezone is produced either by a targeted solidification rate during castingof the running roller 8 or by remelting and then preferably forms thegradient material SoGGH. Consequently, the result is a running roller 8in which a carbide zone 33 has been formed in the region of the surfacesections 12, while the remaining zones and regions of the running roller8 consist of cast steel with unchanged properties. TABLE Preferredmaterial pairings Running roller Piston foot disk Inserts of hard metal,Solid material or inserts e.g. G20, GC37, GC20 comprising a) ceramic,e.g. Si₃N₄ ceramic b) chilled cast iron, e.g.   SoGSH c) Cermet Solidprecision-cast Solid material or inserts material, e.g. GX- comprising210WCr13 H a) ceramic, e.g. Si₃N₄ ceramic b) hard metal, e.g. G20 c)Cermet Solid cast carbide Solid material or inserts material, e.g.chilled comprising cast iron SoGGH a) ceramic, e.g. Si₃N₄ ceramic b)hard metal, e.g. G20 c) Cermet Solid material Solid material or insertscomprising sintered comprising tool steel, e.g. a) ceramic, e.g. Si₃N₄ceramic ASP23, b) hard metal, e.g. G20 comprising C, Cr, Mo, c) CermetV-alloyed nitriding d) cast carbide material, e.g. steel   SoGGH

In each case one or more transverse grooves 36 may be formed in theregion of the flat sections 12 of the running roller 8, as can be seenmost clearly from FIG. 6. As can be seen from FIG. 7, the transversegroove 36 is arranged in the center of a depression 29, forming a grooverun-out, in the flat section 12. The depression 29 is formed by twoplanes arranged at an angle with respect to the flat section 12, withthe transverse groove 36 at their intersection line. The depressionangle γ of the depression 29 is, for example, less than 15 degrees. Thetransition from the depression 29 to the flat section 12 is rounded witha radius R₄ of preferably less than or equal to 1 mm. The radius R₄ isproduced for example by grinding. Fuel can accumulate in this transversegroove 36 or depression 29, which acts as a build-up gap, which fuel, onaccount of the sliding velocity between the flat sections 12 of therunning roller 8 and the piston footplate 18, promotes the formation ofa hydrodynamic sliding film, thereby reducing the wear to the slidingsurfaces.

In the embodiments shown in FIG. 2 to FIG. 4, those parts which remainthe same as and have the same action as in the example shown in FIG. 1are denoted by the same reference designations. By contrast, in theexample shown in FIG. 2, the piston footplate 18 is held on theassociated piston 16 by a plate holder 38. The piston footplate 18, onits surface facing the piston 16, has a circular recess 40, in which thespherically shaped end 42 of the piston 16 engages, coming into contactwith the base of the recess 40. The plate holder 38 is locked on thepiston 16 by means of a circlip 46 engaging in a groove 44 in the piston16. A circular insert 30 made from one of the wear-resistant materialsdescribed above is held in a recess 48 of complementary shape in thepiston footplate 18, for example by cohesive bonding, in particular bysoldering. As can be seen from FIG. 2 a, the insert 30 is provided atthe edge side, on its surface 31 facing the running roller 8, with anangled run-out 35, the run-out angle α amounting to approximately 15degrees. Furthermore, the transition between this surface 31 and therun-out 35 is rounded with a radius R₂ of approx. 2 mm. The transitionbetween the run-out 35 and the edge surface 37 of the insert 30 is alsorounded by means of a radius R₁ of less than or equal to 1 mm.

Similarly to the flat sections 12 of the running roller 8, the inserts30 of the piston footplate 18 preferably have at least two grooves 50which cross one another, as can be seen most clearly from FIG. 3. Onaccount of the grooves 50 being arranged so as to cross one another,there is a high probability that, with regard to the piston footplate 18which can rotate with respect to the plate holder 38, one of the grooves50 will be oriented transversely with respect to the direction ofmovement, in order to promote the formation of a hydrodynamiclubricating film. The grooves 50 are preferably produced by pressing.This results in a lower notch effect compared to chip-forming processes,since the material fibers are not severed. As can be seen from FIG. 2 b,the grooves 50 are each arranged in the center of a depression 39,forming a groove run-out, in the surface 31. The depression is formed bytwo planes arranged at an angle with respect to the surface 31, with therespective groove 50 located at the intersection line of these planes.The depression angle β of the depression 39 is, for example 5 degrees.The transition between the depression 39 and the surface 31 is roundedwith a radius R₃ of preferably less than or equal to 1 mm.

In the exemplary embodiment shown in FIG. 4, the piston footplate 18consists entirely of one of the wear-resistant materials mentioned aboveand is fitted into the passage hole 52 in an annular bush 54 whichconsists of steel. The connection between the annular bush 54 and thepiston footplate 18 is preferably produced by soldering. Of course,there are also other conceivable options for arranging wear-resistantmaterial on the mutually associated sliding surfaces 12, 28 of therunning roller 8 and piston footplate 18.

1-8. (canceled)
 9. A radial piston pump (1) for high-pressure fuelgeneration in fuel injection systems of internal combustion engines, inparticular in a common rail injection system, having a drive shaft (4)which is mounted in a pump casing (2) and has an eccentric shaft section(6) on which a running roller (8) is mounted, and having preferably aplurality of pistons (16), which are arranged in a respective cylinder(14) radially with respect to the drive shaft (4) and each have a pistonfootplate (18), which makes contact with the circumferential surface(10, 12) of the running roller (8), at their ends facing the runningroller (8), wherein at least that surface (28, 31) of the pistonfootplate (18) which is in contact with the circumferential surface (10,12) of the running roller (8) consists of hard metal, a cast carbidematerial, or cermet.
 10. The radial piston pump as claimed in claim 9,wherein the piston footplate (18), on its surface (31) facing therunning roller (8), bears at least one insert (30) made from hard metal,from a cast carbide material or from cermet.
 11. The radial piston pumpas claimed in claim 9, wherein the hard metal contains G20, GC37 or GC20and has a surface roughness R_(z) of between 0.3 μm and 1.0 μm.
 12. Theradial piston pump as claimed in claim 9, wherein the cast carbidematerial contains a chilled cast iron material, in particular GGH orSoGGH, and has a surface roughness R_(z) of between 0.5 μm and 2.0 μm.13. The radial piston pump as claimed in claim 9, wherein the pistonfootplate (18), on its surface (31) facing the running roller (8), hasat least two grooves (50) which cross one another.
 14. The radial pistonpump as claimed in claim 13, wherein one such groove (50) is in eachcase arranged in the center of a depression (39), forming a grooverun-out, in the surface (31).
 15. The radial piston pump as claimed inclaim 9, wherein the surface of the piston footplate (18) and/or of therunning roller (8) has a surface roughness R_(z) of between 0.15 μm and2 μm.